A seismic investigation of the lithosphere of the Gregory rift

Description

During 1976 and the first six months of 1977, the Department of Geological Sciences at Durham University maintained networks of temporary seismic stations over the southeast flank of the Kenya dome and in the central section of the Gregory rift. At each station, signals from local and teleseismic events were recorded from a three component set of seisometers onto magnetic tape. Recorder generated timecode, and BBC GMT pips recorded alongside, enable reproduced seismograms to be timed accurately. Waveform matching of replayed teleseismic P-wave arrivals enabled relative onset times to be obtained with great accuracy. Delay times were obtained for each of the 24 stations, also with high relative accuracy. It is shown that the significantly larger delay times obtained for stations near the culmination of the dome must be due to the presence of anomalously low P-wave velocity material in the upper mantle. A localised trough in the pattern of delay times over the rift and coincident with the positive axial Bouguer anomaly is shown to be due to the presence of anomalously high P-wave velocity material within the crust. Preliminary interpretations assume horizontal layering beneath each station. Flat bottomed models, assuming a uniform anomalous zone velocity of 7.5 km/sec are derived for profiles running southeast over the flank of the dome and across the rift. Interpretations for the flank show a sharply increasing thickening of the anomalous zone towards the rift, with a secondary thickening near or under Mt. Kilimanjaro. The rift profile shows that the anomalous zone penetrates the crust to within about 20 km of the surface. A depth of 120 km is deduced for the base of the anomalous zone, but this may be in error due to systematic error in the baseline of station delays. To circumvent the significant errors associated with the assumption of horizontal layering, a three-dimensional ray tracing technique is devised. Flat bottomed models are derived assuming uniform anomalous zone velocities of 7.5 and 7.0 km/sec. The 7.0 km/sec model shows a thinner and shallower anomalous zone, but the overall shapes of these models are in good agreement with the preliminary models. Deficiences in the ray tracing technique are discussed and it is shown that the parameters characterising the three-dimensional models are not well controlled. Suggestions are made for improving the technique. The models are all consistent with the theory that upward perturbation of the 1ithosphere-asthenosphere boundary, giving rise to magmatic activity, thinning of the lithosphere and domal uplift, is the primary cause of rifting.